Systems and Methods for Cascode Switching Power Amplifiers

ABSTRACT

Example embodiments of the invention may provide systems and methods for a power amplifier. The systems and methods may include a first common-source device having a first source, a first gate, a first drain, and a first body, where the first source is connected to the first body, and wherein the first gate is connected to an input port. The systems and methods may further include a second common-gate device having a second source, a second gate, a second drain, and a second body, where the second source is connected to the first drain, where the second source is further connected to the second body, and where the second drain is connected to an output port.

FIELD OF INVENTION

Embodiments of the invention relate generally to cascode poweramplifiers.

BACKGROUND OF THE INVENTION

Power amplifiers (PAs) may be utilized in radio front-ends and otherwireless devices/applications. Because these PA consume a large amountof power, it may be desirable to utilize a PA with high efficiency.However, designing PAs such as complementary metal oxide semiconductor(CMOS) PAs with high efficiency and high output power is a challengingtask. Accordingly, there is a need for high power, high efficiency poweramplifiers.

BRIEF SUMMARY OF THE INVENTION

Example embodiments may be directed to a high efficiency power amplifiersuch as a high efficiency complementary metal oxide semiconductor (CMOS)cascode switching power amplifier. A high efficiency amplifier inaccordance with an example embodiment of the invention may include CMOScascode devices and load networks. According to the example CMOS cascodestructure, a body of common-gate device may be tied to the source ofcommon-gate device (BS-cascode). In an OFF state, the amount of leakagecurrent flowing through the common-gate device may be relatively small.Therefore, the BS-cascode structure may minimize the power loss due toleakage current in subthreshold/weak inversion region in the CMOScascode device. According to an example embodiment of the invention, theBS-cascode structure may be incorporated in a variety of types ofswitching power amplifiers.

According to an example embodiment of the invention, there is a systemfor a power amplifier. The system may include a first common-sourcedevice having a first source, a first gate, a first drain, and a firstbody, where the first source is connected to the first body, and wherethe first gate is connected to an input port, and a second common-gatedevice having a second source, a second gate, a second drain, and asecond body, where the second source is connected to the first drain,where the second source is further connected to the second body, andwhere the second drain is connected to an output port.

According to another example embodiment of the invention, there is amethod for a power amplifier. The method may include providing a firstcommon-source device having a first source, a first gate, a first drain,and a first body, where the first gate is operative as an input port,and providing a second common-gate device having a second source, asecond gate, a second drain, and a second body, where the second drainis operative as an output port. The method may further includeconnecting the first source to the first body, connecting the secondsource to the second body, and stacking the first common-source deviceand the second common-gate device by connecting the first drain to thesecond source.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is a block diagram for a power transmitter system including aswitching power amplifier, according to an example embodiment of theinvention.

FIG. 2 is a schematic diagram of a BS-CMOS cascode switching poweramplifier system with impedance transformation network, according to anexample embodiment of the invention.

FIG. 3 is a schematic diagram of an example CMOS BS-cascode class-Epower amplifier system, according to an example embodiment of theinvention.

FIG. 4 is an equivalent circuit model of an example CMOS BS-cascodeclass-E power amplifier system, according to an example embodiment ofthe invention.

FIG. 5 is a schematic diagram of an example CMOS BG-cascode class-Epower amplifier system, according to an example embodiment of theinvention.

FIG. 6 is an equivalent circuit model of an example CMOS BG-cascodeclass-E power amplifier system, according to an example embodiment ofthe invention.

FIG. 7 is the voltage waveforms at the drain and source of thecommon-gate device and the gate-to-source voltage V_(GS) of thecommon-gate device, according to an example embodiment of the invention.

FIG. 8 is a graph of example measurement results of a BS-cascode poweramplifier structure in accordance with an example embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Example embodiments of the invention now will be described more fullyhereinafter with reference to the accompanying drawings, in which some,but not all embodiments of the invention are shown. Indeed, theseinventions may be embodied in many different forms and should not beconstrued as limited to the embodiments set forth herein; rather, theseembodiments are provided so that this disclosure will satisfy applicablelegal requirements. Like numbers refer to like elements throughout.

FIG. 1 illustrates a switching power amplifier (PA) system 100 that mayinclude an input port 101, an optional driver amplifier stage 102, oneor more switching power amplifiers 103, and an optional impedancetransformation network 104, according to an example embodiment of theinvention. According to an example embodiment of the invention, thedriver amplifier stage 102 may receive an input such as a basebandsignal or an RF signal from the input port 101, and may generate anoutput to drive the switching power amplifier 103. As shown in FIG. 1,the switching power amplifier 103 may be powered via a supply voltageport 106 (Vdd). The switching power amplifier 103 may then provide anamplified output signal to the impedance transformation network 104,which matches the output impedance of the power amplifier 103 to a loadimpedance at the output port 105. According to an example embodiment ofthe invention, the load may be a switch, a multiplexer, a filter, anantenna, or yet another type of load. According to an example embodimentof the invention, the load impedance may be 50 ohms. Where the loadimpedance is 50 ohms, the impedance transformation network 104 maytransform the output impedance of the switching power amplifier 103 to50 ohms, according to an example embodiment of the invention.

FIG. 2 illustrates a schematic diagram of a CMOS cascode switching poweramplifier system utilizing a BS (body-source) cascode configuration,according to an example embodiment of the invention. As shown in FIG. 2,the power amplifier system 200 may include a BS-cascode switchingamplifier 220 that is in communication with and an optional impedancetransformation network 216. According to an example embodiment of theinvention, the impedance transformation network 216 may be a 1:ntransformer that is operative to match the output impedance of theswitching power amplifier 220 to the load impedance (e.g., 50 ohms) ofthe output load 217 (Pout).

Still referring to FIG. 2, the BS-cascode switching amplifier 220 may bea CMOS BS-cascode switching amplifier that includes a firstcommon-source device or transistor 202 (M₁) having a first source 205, afirst gate 204, a first drain 203, and a first body 206, according to anexample embodiment of the invention. Likewise, the CMOS BS-cascodeswitching amplifier 220 may also include a second common-gate transistoror device 208 (M₂) having a second source 211, a second gate 210, asecond drain 209, and a second body 212, according to an exampleembodiment of the invention.

According to an example embodiment of the invention, the firstcommon-source device 202 (M₁) may be connected in series with the secondcommon-gate device 208 (M₂) to reduce the voltage stress or burden thatmay otherwise be borne by a single device. According to an exampleembodiment of the invention, the devices 202 (M₁), 208 (M₂) may beconnected in series by connecting the first drain 203 of the firstcommon-source device 202 (M₁) to the second source 211 of the secondcommon-gate device 208 (M₂). Additionally, a body-source amplifierconfiguration may also be applied to the cascode switching amplifier 220to reduce or minimize leakage current flow in order to increase theefficiency of the amplifier 220. With a body-source amplifierconfiguration in accordance with an example embodiment of the invention,the first source 205 may be connected to the first body 206 of the firstcommon-source device 202 (M₁), and the second source 211 may beconnected to the second body 212 of the second common-gate device 208(M₂). Further, the first source 205 of the first common-source device202 (M₁) may be connected to ground (GND) while the second gate 207 ofthe second common-gate device 208 (M₂) may be connected to a gate biasport 207 (Vg). According to an example embodiment of the invention, aninput port 201 (Pin) may be provided at the first gate 201 of the firstcommon-source device 202 (M₁).

Still referring to FIG. 2, an RF choke 213 may be provided between apower supply 214 (Vdd) and the second drain 209 of the secondcommon-gate device 208 (M₂). The RF choke 213 may be operative to feedDC power to the drain 209. According to an example embodiment of theinvention, the RF choke 213 may be selected to be large enough such thatthe current though the second drain 209 may be substantially constant.Additionally, an output port of the cascode switching amplifier 220 maybe provided at the second drain 209 of the second common-gate device 208(M₂) and connected to a load network 215. According to an exampleembodiment of the invention, the load network 215 may be operative toperform a switching operation between the output port and an impedancetransformation network 216. The structure of the load network 215 may bedependent on the characteristics (e.g., class-D, E, or F, etc.) of theswitching power amplifier 220. For example, Class-D, E, and F switchingmode power amplifiers (PAs) may require the load networks to performtheir own switching operations. In this case, Class-D PAs may requireLCR resonators for their operation. Likewise, Class-E PAs may requireone or more LCR branches, and class-F PAs may require several lumpedelements to perform harmonic termination. In switching operation, one ormore devices may be heavily overdriven, and the load networks acting asswitching networks may convert DC energy to RF energy, perhaps at 100%efficiency, according to an example embodiment of the invention.

It will be appreciated that the transistors 202 (M₁), 208 (M₂) may bemetal oxide semiconductor field-effect transistors (MOSFETs), accordingto an example embodiment of the invention. However, it will beappreciated that other field effect transistors FETs may likewise beutilized without departing from example embodiments of the invention.

FIG. 3 illustrates a circuit diagram of a CMOS BS-cascode class-E poweramplifier 300, according to an example embodiment of the invention. Asshown in FIG. 3, a BS-cascode amplifier configuration may be implementedusing a first common-source transistor or device 302 (M₁) and a secondcommon-gate transistor or device 308 (M₂), according to an exampleembodiment of the invention. The first device 302 (M₁) may include afirst source 305, a first gate 304, a first drain 303, and a first body306, and the second device 308 (M₂) may have a second source 311, asecond gate 310, a second drain 309, and a second body 312. According toan example embodiment of the invention, the first device 302 (M₁) may inseries with the second device 308 (M₂) by connecting the first drain 303to the second source 311. Additionally, the BS-cascode amplifierconfiguration may be implemented by connecting the first source 305 tothe first body 306 as well as the second source 31 to the second body312.

In FIG. 3, the common-source device 302 (M₁) may have its source 305connected to ground while the common-gate device 308 (M₂) may have itsgate 310 connected to a gate bias port 307. Furthermore, as shown inFIG. 3, the output port at the second drain of the second device 308(M₂) may be connected to a load network 315. According to an exampleembodiment of the invention, the load network 315 may be an L-C-Rnetwork that may be operative for class-E switching conditions. Forexample, a class-E load network 315 may comprise a series combination ofa capacitive component 315, an inductive component 316, and a resistivecomponent 317. According to an example embodiment of the invention, thecomponents 315, 316, and 317 may be implemented using lumped components.

FIG. 4 illustrates an equivalent circuit model of the BS-cascode poweramplifier of FIG. 3, according to an example embodiment of theinvention. As shown in FIG. 4, the common-source device 302 (M₁) may berepresented as a switch 401 having an OFF state and an ON state,according to an example embodiment of the invention. Likewise, thecommon-gate device 308 (M₂) may be represented as a switch 404 having anOFF state and an ON state, according to an example embodiment of theinvention. According to an example embodiment of the invention, theON-state resistance 402 (r_(on1)) of common-source device 302 (M₁) maybe connected in series with the ON-state resistance 405 (r_(on2)) ofcommon-gate device 308 (M₂). Still referring to FIG. 4, the outputcapacitance 403 of the common-source device 302 (M₁) may be the sum ofthe drain-to-body capacitance C_(db1) of device 302 (M₁) and thegate-to-source capacitance C_(gs2) of device 308 (M₂). The outputcapacitance 403 may be connected in series with the output capacitance406 of device 308 (M₂), which may be the drain-to-body capacitance 406(C_(db2)) of device 308 (M₂).

It will be appreciated that in some embodiments it may be desirable toreduce or eliminate the leakage current in the common-gate device 308(M₂) when the BS-cascode amplifier is in the OFF state in order toincrease the efficiency of the BS-cascode amplifier. According to anexample embodiment of the invention, the leakage current of thecommon-gate device 308 (M₂) may be reduced or eliminated by utilizingthe BS-cascode amplifier configuration in accordance with an exampleembodiment of the invention, which places the source of the common-gatedevice 308 (M₂) at a higher voltage than the gate voltage (V_(G)) lessthe threshold voltage (V_(TH2)) of the common-gate device 308 (M₂).

More specifically, with the BS-cascode amplifier configuration, thesource to substrate capacitance C_(sb1) of device 302 (M₁) may beeliminated since the source of device 302 (M₁) may be connected to thebody of device 302 (M₁). Accordingly, the output capacitance 403 for thecommon-source device 302 (M₁) may be reduced through the elimination ofthe source to substrate capacitance C_(sb1). In this way, during an OFFstate for the BS-cascode amplifier in FIG. 4, AC current flows throughthe series path defined by the output capacitances 403, 406, and placesthe source voltage V_(S) of common gate device 308 (M₂) at a higherpotential than the gate voltage V_(G) less the threshold voltage V_(TH2)of device 308 (M₂) (e.g., V_(G)−V_(TH2)). Thus, in the OFF state, theleakage current may be eliminated or reduced within the common-gatedevice 308 (M₂). According to an example embodiment of the invention,the elimination or reduction of the leakage current in the common-gatedevice 308 (M₂) may be operative to improve the efficiency of theBS-cascode amplifier.

FIG. 5 illustrates a circuit diagram of the example CMOS body-ground(BG) cascode class-E power amplifier, according to an example embodimentof the invention. In particular, the circuit diagram of FIG. 5illustrates a BG-cascode amplifier configuration that includes a firstcommon-source device 502 having a first source 505, a first gate 504, afirst drain 503, and a first body 506, and a second common-gate device508 having a second source 511, a second gate 510, a second drain 509,and a second body 512. According to an example embodiment of theinvention, the common-source device 502 may be connected in series withthe common-gate device 508 by connecting the first drain 503 to thesecond source 511. According to an example embodiment of the invention,the series connection of the common-source device 502 and thecommon-gate device 508 may reduce the voltage stress that wouldotherwise be borne by any one device.

In FIG. 5, the BG-cascode amplifier configuration may include connectingboth the first body 506 of the first common-source device 502 and thesecond body 512 of the common-gate device 508 to ground. According to anexample embodiment of the invention, the gate 504 of the common-sourcedevice 502 may be connected to an input port 501 (Pin) while the gate510 of the common-gate device 508 may be connected to a gate bias port507 (Vg). According to an example embodiment of the invention, theoutput port of the BG-cascode amplifier may be provided at the seconddrain 509 of the second common-gate device 508 and connected to a loadnetwork. According to an example embodiment of the invention, the loadnetwork may be comprised of L-C-R components, including a capacitivecomponent 515 (C_(S)), an inductive component 516 (L_(S)), and aresistive element (R_(L)), that are operative with class-E switchingconditions.

FIG. 6 illustrates an equivalent circuit model of BG-cascode class-Epower amplifier, according to an example embodiment of the invention. Asshown in FIG. 6, the common-source device 502 may be represented as aswitch 601 having an OFF state and an ON state, according to an exampleembodiment of the invention. Likewise, the common-gate device 508 may berepresented as a switch 604 having an OFF state and an ON state,according to an example embodiment of the invention. According to anexample embodiment of the invention, the ON-state resistance 602(r_(on1)) of common-source device 502 may be connected in series withthe ON-state resistance 605 (r_(on2)) of common-gate device 508.

Still referring to FIG. 6, the output capacitance 603 of thecommon-source device 502 may be the sum of drain-to-body capacitanceC_(db1) of device 502, the gate-to-source capacitance C_(gs2) of device508, and the source-to-body capacitance C_(sb2) of device 508. Theoutput capacitance 603 may be separated from the output capacitance 606of the common-gate device 508, which may be comprised of thedrain-to-body capacitance (C_(db2)) of device 508, according to anexample embodiment of the invention.

According to an example embodiment of the invention, the source voltageof the common-gate device 508 may rise up to V_(G)−V_(TH2) in an OFFstate. Because the large voltage difference between drain and source ofdevice 508, the device 508 may be in the subthreshold/weak inversionregion, leakage current may flow, thereby resulting in power loss.Accordingly, to minimize this leakage current, a tuning inductor may beconnected to the source of device 508 to increase the source voltage ofdevice 508 and minimize the leakage current. However, the design of theBG-cascode amplifier may require additional space to accommodate thedesign of the tuning inductor.

Simulation and Experimental Results

FIG. 7 shows the simulated voltage waveforms at the drain and source ofa common-gate device and gate-to-source voltage V_(GS) of thecommon-gate device for both BG-cascode and BS-cascode amplifiers,according to an example embodiment of the invention. Each of the cascodestructure has a stack configuration of 0.18-μm common-source and 0.35-μmcommon-gate, according to an example embodiment of the invention. In theOFF state of a common-source device, the source voltage of thecommon-gate device in the BS-cascode amplifier may be larger than thatfor a BG-cascode amplifier. According to an example embodiment of theinvention, the source voltage of the common-gate device in theBG-cascode amplifier may increase due to charging output capacitor ofcommon-gate device until the common-gate device goes into subthresholdregion. The source voltage may rise to V_(G)−V_(TH2), according to anexample embodiment of the invention. The threshold voltage, V_(TH), of0.35-um device may be around 1 V when the body is grounded and source isaround 2.5 V as shown in FIG. 7. According to an example embodiment ofthe invention, the gate-to-source voltage V_(GS) of the common-gatedevice in BG-cascode may be V_(TH2) in OFF state of the common-sourcedevice, since voltage variation of the source of the common-gate devicemay is resulted from the leakage current flowing into large outputcapacitor of the common-source device and almost same. Though theleakage current is small, the power loss may be large because of largevoltage difference between source and drain of the common-gate device.On the other hand, the source voltage of the common-gate device in theBS-cascode may be increased in the OFF state of common-source device,since AC current still flows through series capacitor connection,C_(db2) and C_(gs2)+C_(db1). In an OFF state, due to the current flow,the source voltage of common-gate device M₂ may exceed more thanV_(G)−V_(TH), as shown FIG. 7. In an OFF state of the common-sourcedevice M₁, the gate-to-source voltage V_(GS) of the common-gate deviceM₂ in the BS-cascode may be smaller than one in BG-cascode amplifier sothat the subthreshold current of the BS-cascode may likewise be smallerthan that in the BG-cascode amplifier. Additionally, the power loss fora BS-cascode amplifier may be minimized without adding an additionalinductor, according to an example embodiment of the invention.

Another aspect of a BS-cascode class-E PA may be a small composition ofparasitic output capacitance. The output capacitance of thecommon-source device M₁ in BS-cascode may be smaller than that in aBG-cascode, as shown in FIG. 4 and FIG. 6. A small output capacitancecan reduce power loss in an OFF state as described herein. Also, smalloutput capacitor of the common-source device M₁ may reduce thetransition time of the common-gate device M₂ from an ON state to an OFFstate, and vice versa. Generally, to minimize power loss due to theON-state resistance r_(on), the device size may need to be increaseduntil the parasitic capacitance seen from the drain of the common-gatedevice M₂ can be incorporated into circuit with the required parallelcapacitance for class-E switching condition. Since the effectivecapacitance from drain of the common-gate device M₂ is series connectionof C_(db2) and C_(gs2)+C_(db1), the total capacitance in BS-cascode maybe smaller than that in BG-cascode. Therefore, a larger device may beused without increasing parasitic capacitance, according to an exampleembodiment of the invention.

FIG. 8 illustrates example measured results for the operation of anexample BS-cascode power amplifier in accordance with an embodiment ofthe invention. These measurement results illustrate the output power andthe power added efficiency of such an example power amplifier. As shownin FIG. 8, the measured results meet the demand of the power amplifiercapability in the frequency range between 1700 MHz 1950 MHz. An outputpower of 30.5 dBm at 1.75 GHz and the corresponding power addedefficiency 45% was obtained with a 3.3 V power supply.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the inventions are not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

1. A system for a power amplifier, comprising: a first common-sourcedevice having a first source, a first gate, a first drain, and a firstbody, wherein the first source is connected to the first body, andwherein the first gate is connected to an input port; and a secondcommon-gate device having a second source, a second gate, a seconddrain, and a second body, wherein the second source is connected to thefirst drain, wherein the second source is further connected to thesecond body, and wherein the second drain is connected to an outputport.
 2. The system of claim 1, further comprising a load networkcoupled to the output port.
 3. The system of claim 2, wherein the poweramplifier is a Class-E amplifier, and the load network includes a seriesconnection of an inductive component, a capacitive component, and aresistive component.
 4. The system of claim 2, further comprising animpedance transformation network, wherein the load network is operableto provide a switching operation between the output port and theimpedance transformation network.
 5. The system of claim 4, whereinimpedance transformation network is connected between the load networkand an output load.
 6. The system of claim 5, wherein the output loadincludes a switch, a multiplexer, a filter, or an antenna.
 7. The systemof claim 1, wherein the input port is operable with a baseband signal ora radio frequency (RF) signal.
 8. The system of claim 1, wherein thefirst common-source device provides a first output capacitance definedby the sum of a gate-to-source capacitance of the second common-gatedevice and a drain-to-body capacitance of the first common-sourcedevice, and wherein the second common-gate device provides a secondoutput capacitance defined by a drain-to-body capacitance of the secondcommon-gate device.
 9. The system of claim 8, wherein during an OFFstate for the power amplifier, AC current flows through the first outputcapacitance and the second output capacitance, thereby increasing asource voltage at the second source of the second common-gate device.10. The system of claim 9, wherein the second common-gate device furtherincludes a gate voltage and a threshold voltage, wherein the sourcevoltage is greater than a difference between the gate voltage at thesecond gate and the threshold voltage of the second common-gate device,thereby minimizing leakage current flow in the power amplifier.
 11. Amethod for a power amplifier, comprising: providing a firstcommon-source device having a first source, a first gate, a first drain,and a first body, wherein the first gate is operative as an input port;providing a second common-gate device having a second source, a secondgate, a second drain, and a second body, wherein the second drain isoperative as an output port; connecting the first source to the firstbody; connecting the second source to the second body; and stacking thefirst common-source device and the second common-gate device byconnecting the first drain to the second source.
 12. The method of claim11, further comprising coupling a load network to the output port. 13.The method of claim 12, wherein the power amplifier is a Class-Eamplifier, and the load network includes a series connection of aninductive component, a capacitive component, and a resistive component.14. The method of claim 12, wherein the load network is operable toprovide a switching operation between the output port and an impedancetransformation network.
 15. The method of claim 14, further comprisingconnecting the impedance transformation network between the load networkand an output load.
 16. The method of claim 15, wherein the output loadincludes a switch, a multiplexer, a filter, or an antenna.
 17. Themethod of claim 11, wherein the input port is operable with a basebandsignal or a radio frequency (RF) signal.
 18. The method of claim 11,wherein the first common-source device provides a first outputcapacitance defined by the sum of a gate-to-source capacitance of thesecond common-gate device and a drain-to-body capacitance of the firstcommon source device, and wherein the second common-gate device providesa second output capacitance defined by a drain-to-body capacitance ofthe second common-gate device.
 19. The method of claim 18, whereinduring an OFF state for the power amplifier, AC current flows throughthe first output capacitance and the second output capacitance, therebyincreasing a source voltage at the second source of the secondcommon-gate device.
 20. The method of claim 19, wherein secondcommon-gate device further includes a gate voltage at the second gateand a threshold voltage for the second common-gate device, wherein thesource voltage is greater than a difference between the gate voltage andthe threshold voltage, thereby minimizing leakage current flow in thepower amplifier.